Improved mechanical properties of magnesium alloy thick plate joint via uniform microstructure by differential double-shoulder friction stir welding

•The Mg alloy thick plate joint was prepared by differential double-shoulder friction stir welding.•The grain size uniformity of the joint was increased by 78.5%.•The tensile strength coefficient of the joint reached 86.4%.•The crack initiation mechanism of the joint was basal slip. The magnesium (Mg) alloy thick plates welded using traditional friction stir welding usually exhibit large temperature gradient and uneven microstructure, which seriously affect the joint quality. In this work, a new technology of differential double-shoulder friction stir welding (DDS-FSW) was innovatively proposed to weld a 20 mm AZ31 Mg alloy thick plate. The microstructure and mechanical properties of the joint were studied. The results indicated that DDS-FSW improved the grain size uniformity and texture consistency of the joint, and as a result the deformation incompatibility of the joint was weakened. The tensile strength of the joint reached 86.4 % of the base material. The crack initiation mechanism of DDS-FSW joint was basal slip caused by “orientated micro-interface”. DDS-FSW provides a new and effective strategy for the Mg alloy thick plates welding.